Seismic energy sources can roughly be classified into sources that produce high-energy, low-frequency (say 2 to 30 Hz) acoustic pulses, and those that produce low-energy, high-frequency (say 600 to 7000 Hz) acoustic pulses. Since the earth attenuates high-frequency signals, only low-frequency sources are used for deep-penetration, low-resolution seismic work. The high-frequency souces are used for shallow-penetration, high-resolution seismic work.
Various low-frequency seismic sources using gas exploders have been proposed in the literature, and several such gas exploders have been successfully employed commercially. Perhaps the most widely used gas exploder is of the type described in U.S. Pat. No. 3,235,027 of L. G. Kilmer. This patent and several others of Kilmer describe a well-known DYNOSEIS.sup.R gun. Also of interest is U. S. Pat. No. 3,361,226 which shows a pair of plates defining a permanent chamber therebetween. The chamber is normally closed by an imperfect side wall seal which opens upon the separation of the plates.
Kilmer's commercially-employed gas exploder essentially comprises a rigid top plate positioned over a rigid bottom plate. A vertical, extensible sidewall between the top and bottom plates enclose a detonation chamber therebetween. The chamber accepts a combustible gas mixture which is periodically ignited. The energy of combustion strikes the bottom plate and extends the side wall. The bottom plate moves relative to the top plate and transmits an acoustic signal into the earth. The gas products of combustion are vented from the combustion chamber by an upwardly-extending exhaust line.
It was found that the known gas detonators having a cylindrical bottom plate are rather inefficient for use over uneven terrains.
Also, for high-resolution seismic profiling, used in shallow seismic work, known commercial acoustic energy sources use transducers for converting electric energy into acoustic energy. Unfortunately, the efficiency of such electrically-operated transducers is very low, on the order of 1% or less. This is so because the primary fuel is typically used to first power a diesel engine that drives an AC alternator whose output is converted into DC energy, which is in turn converted by the transducer to acoustic energy. Thus, even a portable, electrically-operated transducer requires bulky and heavy auxiliary equipments which make the overall system inefficient and limited for special applications. The main advantage of such electric transducers is that they can be operated at high repetition rates.
Accordingly, it is an important object of the present invention to provide an efficient, high-frequency, land seismic energy source which is especially adapted for working over uneven and rough terrains.
It is a further object of this invention to provide a gas exploder of the type described in said co-pending applicaton, but whose bottom plate's side wall tapers for the purpose of focusing the transmitted energy into the ground.
To obtain a relatively-high repetition rate, it is essential that the spent gases be flushed out from the combustion chamber as fast as possible, yet without creating acoustic noise that might interfere with the detected reflected seismic signals.
In accordance with another aspect of this invention, the gaseous products of combustion are vented from the detonator into a reservoir filled with liquid, such as water. The mass of the water adds to the mass of the bottom plate for better coupling with the ground, and the water muffles the exhaust gases.